The 351C 4V heads sure do put out the CFMs, but the low end torque has always been a problem for circle track racers and street rodders. Actually by putting in the inserts the air flow and velocity is so improved that the drag racers have been impressed. The idea of filling the bottom of the intake ports started way back when Bob Glidden was drag racing. Oops, that just aged me a smudge. Well, anyways, the air flow likes to run down the top of the port. So why not block off the part that is not working and increase the low end torque at the same time?
Works for me.
The kit contains 8 aluminum castings. The castings are glued into position with the Miller-Stephenson glue and left to sit overnight. Then you drill and tap and insert a button head bolt from the bottom side of the intake runner. Now that the insert is secure, you need to put it up on milling machine and remove all extra material that sticks out of the port.
If you are careful and use 2 C-Clamps on each port, you will find that it is an excellent fit and if you were to port match, 5 minutes with a 3/8” sanding roll and you are done. The reason we use 2 C-clamps is a “live and learn” experience and we found out that blending the back part (which is always the hardest), you know, the part that goes around to the valve,(short side radius) is always a kicker. By carefully installing the inserts, you have done 95% of all work and blending is minimal.